The present invention relates to a method of producing a fatty acid lower alkylester which can be effectively used as a fuel oil for diesel engines, for instance, through a transesterification of triglyceride contained in a variety of fats and oils, such as rape oil, sesame oil, soybean oil, corn oil, sunflower oil, palm oil, palm kernel oil, coconut oil and safflower oil, in particular, waste edible oils that were spent, with an alcohol.
Large quantities of edible fats and oils as mentioned above are used. Although a certain part of spent oils (aforementioned waste edible oils) is recycled as a raw material for producing soap, for example, they are mostly burned together with other combustible wastes or disposed in landfills together with incombustible wastes in actuality.
It has conventionally been known that a fatty alkylester could be obtained by subjecting any of monoglyceride, diglyceride and triglyceride, major constituents of plant oils, to a transesterification with an alcohol (Handbook of Organic Chemistry, published by Gihodo Shuppan, 1988, pp. 1407-1409). Various attempts have thus far been made to produce an alkylester usable as a diesel engine fuel oil from plant oils or fats, or from waste edible oils or fats, as disclosed in Japanese Unexamined Patent Publication Nos. 7-197047 and 7-310090, for example. It has not been possible however to obtain any alkylester complying with current requirements of the Japanese Industrial Standards (JIS) imposed on light oil products by such prior art technology.
Since the transesterification is an equilibrium reaction, a conventional approach to achieving a high yield is to shift the equilibrium state so that a reaction system includes a larger amount of the desired product by using a large quantity of alkylalcohol which is one of raw materials or to removing glycerin which is produced as a result of a side reaction. It is generally understood that gas-phase reaction is advantageous compared to liquid-phase reaction in the transesterification. To add, it is a common practice in the transesterification to use a catalyst for increasing the reaction rate.
In typical industrial processes based on the transesterification, acid catalysts are often used for the production of acetic acid, higher fatty acids, unsaturated carboxylic acids, and so on. For example, protonic acids such as sulfuric acid and phosphoric acid are used as catalysts for the esterification of nonaromatic carboxylic acids while boric acid and phosphoric acid are used as catalysts for the esterification of phenolic acids. Such processes are basically performed in a homogeneous reaction system in which a catalyst is dissolved in a reaction solution. Thus, there exists a problem that it is difficult to separate and recover the catalyst from a liquid end product.
Solid acid catalysts are also used in many cases. For example, SO4.sup.2- --TiO2, TiO2--SiO2, Al2(SO4)3/SiO2.Al2O3 and sulfonic acid type ion exchange resin are used in the transesterification of terephthalic acid or methacrylic acid. Heteropoly acid is also regarded as a good esterification catalyst. It is known for its ability to be carried by silicon dioxide (SiO2) and activated carbon and exhibit higher activity than SiO2--Al2O3 or solid phosphoric acid as a gas-phase catalyst. Clay minerals are also used as catalysts. One of advantageous features of these solid acid and mineral catalysts is that they contribute to the simplification of reaction equipment. This is because no special facility is required for separating them from liquid end products. These industrial catalysts have one serious drawback that their activity is low in the transesterification of fats and oils and it has not been possible so far to use them in the aforementioned processes in an industrial scale.
Another previous approach to the use of solid acid catalysts in transesterification of fats and oils is disclosed in Japanese Unexamined Patent Publication No. 6-313188, in which single or composite metallic oxide, metallic sulfate, metallic phosphate, immobilized acid carried by or fixed to a carrier, natural minerals and compounds having a layer structure, solid heteropoly acid, superstrong acid, synthetic zeolite and ion exchange resin are cited as examples of applicable catalysts. However, such potential catalysts of this approach have low catalytic activity in the transesterification of fats and oils as was the case with the earlier-mentioned conventional approaches, and it has been necessary to increase the ratio of a solid acid catalyst in the reaction system or prolong the reaction time in order to achieve high yields.
Also used in the transesterification are basic catalysts, and it is known that metallic alcoholates are effective basic catalysts. Generally used examples of the metallic alcoholates include sodium alcoholate and potassium alcoholate. Other examples used as the metallic alcoholates are sodium hydroxide, potassium hydroxide and sodium carbonate which exhibit high activity in the transesterification of fats and oils. The previously used basic catalysts act only when they are dissolved in a reaction solution like the aforementioned acid catalysts, however. Thus, they remain dissolved in liquid end products and there still exists the problem associated with the difficulty of separating and recovering the catalysts from the liquid end products.
Still another previous approach is to use solid basic catalysts in the transesterification. As an example, Japanese Unexamined Patent Publication No. 62-218495 proposes the use of an ion exchange resin containing a base of the amine group as a solid basic catalyst. Although this approach is not associated with the problem related to the separation and recovery of the catalyst in principle, it involves the use of an excessive amount of alcohol and the reaction is performed at a triglyceride content of 0.1 to 3 wt %. Accordingly, catalytic activity is extremely low and the reaction temperature is limited to 60.degree. C. or less from the viewpoint of the durability of the ion exchange resin and, as a consequence, the approach is not really practical.